The present invention, while applicable generally to the preheating of combustion supporting gases, is particularly suited for furnaces used in the production of glass spheres.
It is well known to produce glass beads or spheres by introducing crushed glass particles into a vertically disposed draft tube. The tube has a source of heat near its lower end, normally a well distributed gas flame produced by a series of burners. As the combustion gases rise, they proceed into an expansion chamber and carry with them the glass particles which become soft, so that by surface tension the particles are shaped into spherical form. Such bead producing systems are discussed in more detail, for example, in U.S. Pat. No. 2,619,776, granted Dec. 2, 1952, to Rudolph H. Potters and in U.S. Pat. No. 2,945,326, granted July 19, 1960, to Thomas K. Wood.
Heretofore, bead producing furnaces of the foregoing type have been provided with air for combustion at ambient temperature. One of the more common prior systems involved the introduction of the combustion air through an opening in the draft tube below the heat source. The draft created by the rising combustion gases drew ambient air into the draft tube where it was mixed with a combustible fluid and ignited.
As the burning gases rose, much of their heat was transferred through the walls of the draft tube and the expansion chamber. Excessive heating of these walls will result in their deterioration as well as the adhesion of molten glass particles thereto, and in some cases it was necessary to develop cooling systems involving the direction of ambient air against the outer walls of the expansion chamber to meet this problem. These arrangements were basically inefficient, however, and the heat loss through the walls of the expansion chamber resulted in a substantial wastage of the available heat energy.